1. Field of the Invention
The present invention relates to a solenoid-driven, partly-open type shutter apparatus (diaphragm shutter apparatus) which is arranged to record a still image of an object on a recording medium by opening a shutter blade assembly.
2. Related Background Art
Such a partly-open type of shutter apparatus is generally arranged to gradually open a shutter blade assembly by the rotation of a motor, energize a control magnet to disengage a clutch when the aperture defined by the shutter blade assembly reaches a predetermined aperture value, and rapidly close the shutter blade assembly by utilizing the spring force of a spring associated therewith. The reason for adopting this arrangement is that a mechanism having a slow opening operation and a fast closing operation is advantageous for stabilizing the amount of exposure.
Another form of arrangement has been adopted in which gears and the like which constitute part of a shutter mechanism are restored to their initial positions by spring forces. There is also a known mechanism in which, after a shutter blade assembly has been closed, gears and the like are restored to their initial positions by reversing a motor.
A common type of control system for causing a motor to rotate to open the above-described shutter blade assembly is usually arranged to provide constant-voltage control (full energization) under which a power source voltage is supplied directly to the motor for the purpose of energization. To meet a demand for the development of a shutter having a higher accuracy, a control system of the type which provides constant-speed control over a shutter opening speed has recently been proposed.
The basic operation of such a partly-open type shutter apparatus is as follows. The partly-open type shutter apparatus serves as a diaphragm and a shutter, and is arranged in such manner that its aperture is gradually opened in association with the opening operation of the shutter blade assembly and, when the quality of light integrated on the surface of a photosensor reaches a predetermined value, a close signal is supplied to the shutter blade assembly and shutter control is in turn executed. However, in practice, if the shutter opening speed changes by the influence of a power source voltage or the like, there will occur an time interval between the application of the shutter close signal and the execution of the shutter control. As a result, the amount of exposure will vary owing to the fact that the aperture becomes excessively open for faster shutter opening speeds and imperfectly open for lower shutter opening speeds.
The variation in the amount of exposure leads to a so-called unevenness of exposure (generally known as the "unevenness of shutter operation"), which may result in a deterioration in the accuracy of shutter operation. The above-described conventional partly-open type of shutter apparatus has the following problems related to the deterioration in the accuracy of shutter operation due to the "unevenness of exposure".
For example, in a system in which a motor driving means is fully energized by applying a power source voltage for the purpose of opening a shutter blade assembly, the speed at which the shutter blade assembly opens changes due to variations in the power source voltage due to, for example, the dissipation of battery power, drifts in a circuit due to changes in environmental conditions, or load changes. It will therefore be difficult to achieve a highly accurate shutter speed.
In general, the running characteristics of the shutter blade assembly of the conventional partly-open type shutter apparatus are selected so that the amount of transmitted light limited by the shutter blade assembly is approximately proportional to the time required for individual shutter blades to run by their predetermined distances. If the variation of the shutter opening speed is made constant, the unevenness of exposure increases in approximately geometrical progression as the brightness of an object increases. For example, if the running time elapsed is represented as "1" with respect to the amount of transmitted light for an aperture of f/16, the running time elapsed is represented as "2" with respect to the amount of transmitted light for an aperture of f/11, "4" for f/8, "8" for f/5.6, and "16" for f/4. As can be seen from FIG. 16, the amount of light transmitted through each aperture diameter is approximately proportional to the time during which light is transmitted during a shutter running operation. In consequence, if the variation of the shutter opening speed is made constant, the unevenness of exposure may be serious in a high-brightness area in a case where it hardly appears in a low-brightness area. Accordingly, it is difficult to realize highly accurate control of a shutter by utilizing the above-described full energization system.
A so-called servo control system for providing constant-speed control over a shutter opening speed will be considered below. In this system, since the shutter opening speed is placed under constant-speed control, such opening speed does not change due to the variation of a power source voltage or variations in environmental conditions and therefore, no substantial variation in the amount of exposure is caused by an overrun occurring after the application of a shutter close signal. Accordingly, it is possible to maintain a high accuracy of shutter operation over the entire brightness area.
However, since it is impossible to provide constant-speed control in the state of full energization, the speed at which the shutter blade assembly is opening under the constant-speed control is naturally slower than the speed in the state of full energization. As a result, the shutter open time required to obtain a particular amount of exposure is long compared to that required for the full energization, as shown in FIG. 17. This means that, if a moving object is to be photographed in a low-brightness area, the constant-speed control system is unfavorable in that the shutter open time becomes long.
Electronic cameras utilizing solid-state image sensors such as CCDs (change-coupled devices) have also recently been developed. The solid-state image sensors for use in the electronic cameras have the problem of dark current inherent therein, and the length of exposure time is proportional to a deterioration in image quality due to the dark current. For this reason, if a partly-open type shutter apparatus utilizing the above-described constant-speed control system is applied to such an electronic camera, a lowering in the shutter opening speed may lead to a critical defect which aggravates a deterioration in image quality due to dark current, such as white scratches.
It is naturally advantageous in terms of shutter opening speeds to supply a power source voltage in the state of full energization. However, if account is taken into the problem of accuracy and the mechanical shock of a shutter-blade-assembly driving system occurring when the power source voltage is high, the above-described system in which the power source voltage is supplied in the state of full energization may not be desirable in terms of the durability of the apparatus.
To solve the above-described problems experienced with the constant-speed control of shutter opening speeds, needless to say, it is desirable that the shutter opening speeds be constantly set to high speeds. However, it is not easy to constantly set the shutter opening speeds to sufficiently high speeds since it is generally difficult to constantly maintain a high power source voltage in an actual camera.
This difficulty will be explained below with reference to FIG. 6 which shows the construction of a partly-open type shutter apparatus for providing conventional constant-speed control. In the apparatus shown in FIG. 6, a control circuit 3 for providing control over every element of the shutter apparatus receives a signal from a photometry circuit 1 for measuring the brightness of an object and a signal from a release signal generation circuit 2 for generating a release signal when detecting a shutter release switch SW (not shown). The control circuit 3 causes a motor driving circuit 7 to drive a motor 8 for opening a shutter blade assembly through a voltage control circuit 5.
After the shutter blade assembly has started its shutter opening operation by the driving force of the motor 8, a photointerrupter 12 detects the motion associated with the shutter blade assembly, and a position detection signal (h) indicative of the position of the shutter blade assembly is supplied to the control circuit 3 through a photointerrupter wave modulation circuit 11. The control circuit 3 analyzes the current shutter opening speed on the basis of the position detection signal (h) relative to the shutter blade assembly, and outputs the voltage control signal (a) and causes the motor driving circuit 7 to increase or decrease the speed of the motor 8 in accordance with whether the current speed is greater or smaller than the target value of the shutter opening speed.
When the shutter opening reaches a predetermined aperture diameter by the above-described operation, the control circuit 3 outputs a drive signal (g) to the magnet driving circuit 9, which in turn activates a magnet 10 to disengage a solenoid clutch (not shown), thereby rapidly restoring the shutter blade assembly to an initial position thereof by the spring force of a spring (not shown) so as to close the shutter blade assembly.
In the above-described circuit arrangement for driving and controlling the partly-open type shutter apparatus, an excessive voltage drop occurs across the voltage control circuit 5, with the result that it is difficult to realize constant-speed control during a high-speed shutter operation in the state of the power source voltage being low.